Ultrafast lasers generate a series of short optical pulses. Temporal separation of the pulses is determined by a round-trip time of light circulating in the resonant cavity of the laser. The inverse of this pulse separation is general termed the laser frequency (F) and is given by an equation: EQU F=c/L.sub.roundtrip =c/2L.sub.cavity (1)
where c is the speed of light in air L.sub.cavity is the actual linear length of the cavity. By way of example for a laser having a two meters (2 m) long linear cavity, i.e, a cavity with a 4 m round-trip length L.sub.roundtrip, F is about seventy-five megahertz (75 MHz).
A passively mode-locked, ultrafast laser, for example, a Kerr-lens mode-locked titanium-doped sapphire (Ti:sapphire) laser, is not limited in terms of a round-trip time, i.e, a frequency at which it can be operated. For a certain average output power, the energy per pulse and the pulse separation is directly proportional to the length of the laser's resonant cavity. As such, if a high energy per pulse or high pulse-separation time is required, it is desirable to operate the laser with as long a resonant cavity as possible.
Unfortunately, in many applications of ultrafast lasers, such as incorporating the laser in a small instrument, a laser having a cavity length of about 2 m or more is simply not practical. A practical length is about thirty centimeters (cm) or less. In certain applications a length of 10 cm may be desirable. To "fold" a 2 m long cavity, using multiple reflections, to obtain a 10 cm longest physical dimension would require more than twenty reflections, i.e., more than forty reflections per round trip in the cavity. Commercially available laser reflectors are typically vacuum deposited by thermal evaporation of layer-forming materials. Such mirrors typically have a maximum reflection of about 99.8%, or, where special precautions are taken to reduce loss, of about 99.9%.
In most ultrafast lasers, a cavity loss in excess of 1.0% would lead to significant loss of output power. For example, in an ultrafast laser having 10% outcoupling a 1% cavity loss (per round trip) equates to about 10% loss of output power. Because of this, even if 99.9% reflecting fold-mirrors were used, more than about ten intra-cavity reflections therefrom, per round-trip, would produce significant output power reduction.
Further, in order to support the ultrashort pulse length characteristic of an ultrafast laser the laser must possess a total negative group velocity dispersion, (negative GVD or NGVD) i.e., the sum of the GVD of the laser gain medium and all cavity components must be negative. In a simple arrangement of a laser cavity and dielectric material therein, such as, a gain medium and a mode locking device, total cavity GVD would be positive, i.e., shorter wavelength light experiences a higher refractive index and lower group velocity and lags behind longer wavelength light. This causes lengthening of a laser pulse each round trip and prevents stable, short-pulse operation.. One means of avoiding this, is to include one or more NGVD devices having collective negative GVD at least equal to and preferably greater than this positive GVD. Furthermore if the laser is to be tunable over a range of wavelengths, the NGVD devices must be effective over that range of wavelengths.
Reflective NGVD devices which have been used with prior art ultrafast lasers include Gires-Tournois Interferometer (GTI) mirrors, and so called "chirped" mirrors, all of which are multilayer dielectric interference layer structures, typically vacuum deposited by thermal evaporation of materials from electron-beam heated, or resistance-heated sources. Reflective NGVD devices are referred to hereinafter as NGVD-mirrors.
A GTI mirror is a multilayer NGVD-mirror including a reflector, comprising a stack of alternating high and low refractive index dielectric layers, each layer having a thickness of one-quarter wavelength at the nominal operating wavelength of the laser, and a single thick "spacer" layer (typically may wavelengths thick) of a dielectric material deposited on the reflector. A partially reflecting multilayer stack may (optionally) be deposited on the spacer layer. A GTI-mirror typically gives a constant negative GVD over only a relatively narrow wavelength range, for example about fifty nanometers (nm).
A so-called chirped mirror is a multilayer stack alternating high and low refractive index dielectric layers, the layers being varied in thickness throughout the stack, to different degrees, about a nominal quarter-wavelength optical thickness at a nominal laser wavelength. This type of mirror may also be termed a simply a negative dispersion mirror (NDM), a term which is hereinafter used to describe any NGVD-mirror structure which is not a GTI-mirror. Such a mirror can provide constant NGVD over a broader band of wavelengths than a GTI mirror, for example, up to about 200 nm with the same GVD. Such a NDM may include as many as forty-five or more layers.
A common goal of all NGVD-mirrors, however designed and named, is to cause longer wavelengths in a given pulse, i.e., in the bandwidth of the pulse, to take a longer time to be reflected than shorter wavelengths in that pulse. This is achieved, in a NDM, primarily by the large total thickness of the multilayer structure. This total thickness may be three or more times the thickness required to provide a simple 99.8% reflecting multilayer mirror. In a GTI-mirror this is achieved by resonant behavior of electric fields in the spacer layer. This resonant behavior exacerbates any inherent losses in the spacer layer.
Either because of a greater total thickness (NDMs) or resonances (GTI-mirrors) more optical losses (scatter and absorption) are generally experienced in prior-art NGVD-mirrors than in simple fold-mirrors. This is one reason why GTI-mirrors and NDMs have been used only to a limited extent in prior art ultrafast lasers. An ability to provide multiple reflections from a negative-dispersion mirror within a resonant cavity is advantageous in designing such a mirror to be effective over a broad band of wavelengths.